What Is a Closure in JavaScript? Test Independent Captured State

The task

Create two independent ticket counters with different starting points. Keep one returned function reference, call it repeatedly and check that the other counter’s state is unaffected.

Runnable Node 22 example

const assert = require('node:assert/strict');

function ticketCounter(start) {
  let next = start;
  return function () { return next++; };
}
const deskA = ticketCounter(40);
const deskB = ticketCounter(90);
const sameA = deskA;
assert.equal(deskA(), 40);
assert.equal(sameA(), 41);
assert.equal(deskB(), 90);
assert.equal(deskA(), 42);
assert.equal(typeof deskA, 'function');
console.log(`A-next=${deskA()}; B-next=${deskB()}`);

Expected output and explanation

Each call to the factory creates its own next binding. deskA and sameA refer to the same returned function and therefore advance the same counter. deskB belongs to a separate factory invocation and begins at ninety.

The factory has already returned when the counters are called, yet their functions can still access their respective next values. That continued access is the closure behavior being tested. The function reference is not the same thing as one numeric result returned by calling it.

Pitfall to check

Captured state can persist longer than expected if callbacks remain reachable. Keep the lifetime and sharing deliberate. This example demonstrates isolated in-memory counters, not a globally unique ticket allocation system across processes.